Dr Mozhgan Yavari
Academic and research departmentsFaculty of Engineering and Physical Sciences, Advanced Technology Institute.
Over the last decade, 2,2 '',7,7 ''-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 '-spirobifluorene (spiro-OMeTAD) has remained the hole transporting layer (HTL) of choice for producing high efficiency perovskite solar cells (PSCs). However, PSCs incorporating spiro-OMeTAD suffer significantly from dopant induced instability and non-ideal band alignments. Herein, a new approach is presented for tackling these issues using the functionality of organometallocenes to bind to Li+ dopant ions, rendering them immobile and reducing their impact on the degradation of PSCs. Consequently, significant improvements are observed in device stability under elevated temperature and humidity, conditions in which ion migration occurs most readily. Remarkably, PSCs prepared with ferrocene retain 70% of the initial power conversion efficiency (PCE) after a period of 1250 h as compared to only 8% in the control. Synergistically, it is also identified that ferrocene improves the hole extraction yield at the HTL interface and reduces interfacial recombination enabling PCEs to reach 23.45%. This work offers a pathway for producing highly efficient spiro-OMeTAD devices with conventional dopants via addressing the key challenge of dopant induced instability in leading PSCs.
Understanding the fundamental properties of metal halide perovskite materials is driving the development of novel optoelectronic applications. Here, we report the observation of a recoverable laser-induced fluorescence quenching phenomenon in perovskite films with a microscopic grain-scale restriction, accompanied by spectral variations. This fluorescence quenching depends on the laser intensity and the dwell time under Auger recombination dominated conditions. These features indicate that the perovskite lattice deformation may take the main responsibility for the transient, show a new aspect to understand halide perovskite photo-stability. We further modulate this phenomenon by adjusting the charge carrier recombination and extraction, revealing that efficient carrier transfer can improve the bleaching resistance of perovskite grains. Our results provide future opportunities to attain high-performance devices by tuning the perovskite lattice disorder and harvesting the energetic carriers.
Recent advances in heterojunction and interfacial engineering of perovskite solar cells (PSCs) have enabled great progress in developing highly efficient and stable devices. Nevertheless, the effect of halide choice on the formation mechanism, crystallography and photoelectric properties of the low-dimensional phase still requires further detailed study. In this work, we present key insights into the significance of halide choice when designing passivation strategies comprising large organic spacer salts, clarifying the effect of anions on the formation of quasi2D/3D heterojunctions. To demonstrate the importance of halide influences, we employ novel neo-pentylammonium halide salts with different halide anions (neoPAX, X = I, Br or Cl). We find that regardless of halide selection, iodide-based (neoPA)2(FA)(n-1)PbnI(3n+1) phases are formed above the perovskite substrate, while the added halide anions diffuse and passivate the perovskite bulk. In addition, we also find the halide choice has an influence on the degree of dimensionality (n). Comparing the three halides, we find that chloride-based salts exhibit superior crystallographic, enhanced carrier transport and extraction compared to the iodide and bromide analogs. As a result, we report high power conversion efficiency in quasi-2D/3D PSCs, which are optimal when using chloride salts, reaching up to 23.35% and improving long-term stability.
Triple cation CsFAMA perovskite films fabricated via a one-step method have recently gained attention as an outstanding light-harvesting layer for photovoltaic devices. However, questions remain over the suitability of one-step processes for the production of large-area films, owing to difficulties in controlling the crystallinity, in particular, scaling of the frequently used anti-solvent washing step. This can be mitigated through the use of the two-step method which has recently been used to produce large-area films via techniques such as slot dye coating, spray coating or printing techniques. Nevertheless, the poor solubility of Cs containing salts in IPA solutions has posed a challenge for forming triple cation perovskite films using the two-step method. In this study, we tackle this challenge through fabricating perovskite films on a caesium carbonate (Cs2CO3) precursor layer, enabling Cs incorporation within the film. Synergistically, we find that Cs2CO3 passivates the SnO2 electron transport layer (ETL) through interactions with Sn 3d orbitals, thereby promoting a reduction in trap states. Devices prepared with Cs2CO3 treatment also exhibited an improvement in the power conversion efficiency (PCE) from 19.73% in a control device to 20.96% (AM 1.5G, 100 mW cm−2) in the champion device. The Cs2CO3 treated devices (CsFAMA) showed improved stability, with un-encapsulated devices retaining nearly 80% efficiency after 20 days in ambient air.